I own a small Chinese bench lathe with model number CQ6125.
These lathes are sold in many different brand names, no clue on who is copying who..
Mine is called a HBM 250×400 but yours may be a Grizzly G0602 or a Optimum D240.

This lathe has 6 spindle speeds which can be changed by selecting a belt configuration.
Therefor I changed the original motor with a 3 phase 230V motor with VFD.
This way the lathe is converted to a variable speed lathe.


The variable speed control should have at least the following features;
– Speed control using a potentiometer in the front-panel (connected to the VFD)
– The motor must be able to run forward and reversed.
– there must be a tachometer in the front-panel.

So the first stop is making a front panel with a display and some switches.

Left + Right switch are anti-vandal momentary switches – Amazon link
Stop switch is a anti-vandal latching switch with LED – Amazon link
RPM readout found at Ebay (4 Digital Red LED Tachometer )

Wiring diagram: Robo Lathe wiring diagram

  IMG_2826  IMG_2855

The standard 3Nm sigle phase motor is replaced by a 3 phase motor.
Choosing the motor can be confusing, and really, maybe this is not even the best of the best yet..
In choosing the motor I wanted the same size (71 size) motor as the original.
This way mounting the motor is a piece of cake.

Since the original motor has a somewhat thicker and longer shaft we could not
drive the spindle directly from the motor. This is why we used a timing pulley.
The timing pulley is a T5 (metric) 22Teeth pulley for the 16mm belt.
We ordered it locally at Stappenmotor.nl but if you can broaths it yourself get this one : Amazon

The motor we used is from the ‘MT Electric Motors’, check out there website for a selection guide.
We have chosen the ‘TN-71-0.75-2-230/400-50-IP55-CLF-B3’ (Sold at DamenCNC)
– Amazon E-motors –

If you want to know, and understand why we came up to this model, check out the video !

IMG_2858  IMG_2856

The VFD used for this project is chosen mainly because i had one laying around.
Any VFD will do, if you at least take the voltage and power consumption into account.
For this project I went with a 3 x 230V motor with a power consumption of 750 Watt.
The VFD used here is obsolete but take a look over here: Amazon link

The only settings I have changed in the VFD are the ramp-speed (slightly reduced, factor 0,5).
Minimum frequency is set at 10 Hz, maximum is set at 90 Hz.

Wiring diagram: Robo Lathe wiring diagram

Test results:  Test results

All information is provided for free, if used, a link back would be nice..

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